1. Field of the Invention
The present invention relates to a nonreciprocal circuit device, and more particularly, to a nonreciprocal circuit device, such as an isolator, for use in a microwave band and a communication device including such a nonreciprocal circuit device.
2. Description of the Related Art
In a mobile communication device such as a portable telephone, a nonreciprocal circuit device, such as an isolator or a circulator, is typically used to transmit a signal in only one direction without allowing the signal to pass in the opposite direction.
In general, such a nonreciprocal circuit device includes a permanent magnet, a center electrode assembly to which a fixed magnetic field is applied by the permanent magnet, and a metal case in which the permanent magnet and the center electrode assembly are disposed.
As shown in FIG. 12, Japanese Unexamined Patent Application Publication No. 2002-76711 discloses a center electrode assembly 201 including a block-shaped microwave ferrite 231, center electrodes 221 to 223, insulating films 226, side electrodes (through-hole electrodes) 224, and a ground electrode 225.
Three pairs of center electrodes 221 to 223 are disposed on a surface 231a (a pole surface) of the ferrite 231, and insulating films 226 are disposed between respective adjacent pairs of center electrodes. Both ends of each of the center electrodes 221 to 223 are each connected, in corners of the ferrite 231, with side electrodes 224 provided on side surfaces 231c of the ferrite 231. One end of each of the center electrodes 221 to 223 is electrically connected via a side electrode 224 to a ground electrode 225 disposed over substantially the entire lower surface 231b. The side electrodes connected to the other ends of the respective center electrodes 221 to 223 define ports P1, P2 and P3. The ports P1 to P3 are used to connect the center electrode assembly 201 to an external circuit. Each of the ports P1 to P3 is isolated from the ground electrode 225 by a gap 228.
The center electrodes 221 to 223 are formed of a conductive material, such as silver, by screen printing or other suitable methods. The side electrodes 224 are formed as follows. First, holes are formed through the ferrite 231 such that the holes extend from the upper surface to the lower surface of the ferrite 231. Conductive paste is then filled in the through-holes or a plated film is formed on the inner wall of each through-hole. Finally, each through-hole is cut at its center.
The insulating films 226 are formed of glass or other suitable material by screen printing or other suitable methods over almost the entire surface 231a of the ferrite 231 except for a peripheral area such that the center electrodes 221 to 223 crossing each other are isolated from each other by the insulating films 226. Because the insulating films 226 are provided merely to insulate the respective layers of the center electrodes 221-223 from each other, a large alignment tolerance is acceptable in patterning of the center electrodes 221-223. Thus, alignment accuracy obtained in usual screen printing techniques is sufficient.
However, in the center electrode assembly 201 disclosed in Japanese Unexamined Patent Application Publication No. 2002-76711, the center electrodes 221 to 223 provided on the surface 231a of the ferrite 231 have a small uniform thickness, and thus, as shown in a circle A in FIG. 13, the side electrodes 224 and the corresponding center electrodes 221-223 are connected with each other via a small contact area. This results in poor connection reliability.
To avoid the problem described above, it has been proposed to increase the thickness over the entire center electrodes 221 to 223 such that the center electrodes 221 to 223 can be connected with corresponding side electrodes 224 via an increased contact area. However, it is difficult to increase the thickness of the entire center electrodes 221 to 223 by screening printing. In addition, the increase in thickness over the entire center electrodes 221 to 223 results in an increase in the total thickness of the center electrode assembly 201, and thus it is impossible to satisfy the requirement for a reduction in thickness of the center electrode assembly 201.